How to accurately gauge speaker sensitivity to match with tube amp?

The Focals though, like a lot of modern speakers, have dips in the mid-bass which can really depress the output of a tube amp.

In the case of tube amps, I'd look more at the impedance curves (or minimum).  60 W is probably enough to drive them to fun levels in terms of volume.

What should I notice in those curves? Just that they don't dip down as you described -- down to... 2 ohms? For how far across the mid bass?

Volume is fine with the speakers -- they get loud without a problem. It's that control, tightness, I'm looking for.

OP:

I wish I had a better answer for you. Ideally, keeping the impedance above 4, as well as not varying too much. You also would like to see minimal peaks.

The more variation, the more the impedance curve takes command of the frequency response.

Based on all of this, I’d be surprised if you liked this combo:

https://www.stereophile.com/content/focal-aria-936-loudspeaker-measurements

but on the other hand, you might like how it sounds. You’ll get a brighter, more airy presentation, but look how broad the under 4 Ohms is. That’s what I’d listen for. For the most neutral presentation from these speakers you are going to want a beefy solid state amp.

With tubes you'll get a lot of color here, but who am I to tell you if you'd like it??

You have a 60 WPC tube amp with KT88s, virtually the same amp I have run for decades. https://systems.audiogon.com/systems/8367 This is a Melody 50 wpc. Before that was an Aronov very similar 50 wpc. Between the two ran four very different speakers JBL L7, Linaeum Model 10, Vandersteen something or other (very briefly) and Talon Khorus. All these speakers were around or a little more than 90 dB. All played plenty loud in that 17x24 room. All sounded great, for what they were. You have plenty of power.  

Don't even ask me what the impedance curves looked at for any of these speakers. I haven't the foggiest. Used to pay attention to such things. Then I realized impedance curves, like frequency response and all the rest, are at best second derivatives. In other words there is the final result, what you hear, and then there are all the things that go into that. The sound you hear is derived from sensitivity, dispersion, impedance, etc. They are not what you want. They are only things we have learned to measure and derive from what really counts, the sound we get.  

Now considering you are having problems I have never had in 30 years you might want to stop and think about it. If a guy has never had a problem in 30 years, always picked great satisfying speakers, what is he doing? How is he doing this? It ain't by looking at curves on charts.

The only measurement you need is sensitivity greater than 90. Then from there you pay attention to listening impressions. Then when you find people describing a sound that matches what you like look for people who are hearing that sound with tube amps. 

This takes no more time than searching through impedance charts. Its no more easy to describe the sound than measure the impedance. Look how many times JA and others go into excruciating detail about the technical aspects of impedance. Then at best all you have is one more extremely crude measure of performance. It tells you nothing about how deep and wide and inviting the sound stage will be.

But on the other hand, maybe you don't care about such things as musical involvement. Maybe you do only care about tiny little technical details like how tight the bass is between 146.9 and 237.8 Hz. If so then, as Roseanne Rosanna Danna used to say, never mind.